Astrid Tasks uses notifications that cannot be cleared by clicking on the "clear" button. Why they did this is a question only the Astrid developers can answer, but my guess is the people from Astrid want to remind you of a task until you actually finish it and/or change its status in Astrid.

By default notifications on Android can be cleared, but an app developer can choose to make a notification not-clearable (if you want technical details on how to do this, go here). If there are any notifications that can be cleared, the clear button is enabled/visible but will not clear any non-clearable notifications when clicked.